linux-kernel - Re: [PATCH 1/9] ARM: get rid of arch_cpu_idle

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Date:	Mon, 27 Jan 2014 10:45:59 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Catalin Marinas <catalin.marinas@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
	linux-pm@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 1/9] ARM: get rid of arch_cpu_idle_prepare()

On Mon, 27 Jan 2014, Russell King - ARM Linux wrote:

> On Mon, Jan 27, 2014 at 01:08:16AM -0500, Nicolas Pitre wrote:
> > ARM and ARM64 are the only two architectures implementing
> > arch_cpu_idle_prepare() simply to call local_fiq_enable().
> > 
> > We have secondary_start_kernel() already calling local_fiq_enable() and
> > this is done a second time in arch_cpu_idle_prepare() in that case. And
> > enabling FIQs has nothing to do with idling the CPU to start with.
> > 
> > So let's introduce init_fiq_boot_cpu() to take care of FIQs on the boot
> > CPU and remove arch_cpu_idle_prepare(). This is now done a bit earlier
> > at late_initcall time but this shouldn't make a difference in practice
> > i.e. when FIQs are actually used.
> > 
> > Signed-off-by: Nicolas Pitre <nico@...aro.org>
> > ---
> >  arch/arm/kernel/process.c | 5 -----
> >  arch/arm/kernel/setup.c   | 7 +++++++
> >  2 files changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> > index 92f7b15dd2..725b8c95e0 100644
> > --- a/arch/arm/kernel/process.c
> > +++ b/arch/arm/kernel/process.c
> > @@ -142,11 +142,6 @@ static void default_idle(void)
> >  	local_irq_enable();
> >  }
> >  
> > -void arch_cpu_idle_prepare(void)
> > -{
> > -	local_fiq_enable();
> > -}
> > -
> >  void arch_cpu_idle_enter(void)
> >  {
> >  	ledtrig_cpu(CPU_LED_IDLE_START);
> > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> > index 987a7f5bce..d027b1a6fe 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -789,6 +789,13 @@ static int __init init_machine_late(void)
> >  }
> >  late_initcall(init_machine_late);
> >  
> > +static int __init init_fiq_boot_cpu(void)
> > +{
> > +	local_fiq_enable();
> > +	return 0;
> > +}
> > +late_initcall(init_fiq_boot_cpu);
> 
> arch_cpu_idle_prepare() gets called from the swapper thread, and changes
> the swapper thread's CPSR. init_fiq_boot_cpu() gets called from PID1, the
> init thread, and changes the init thread's CPSR, which will already have
> FIQs enabled by way of how kernel threads are created.
> 
> Hence, the above code fragment has no effect what so ever, and those
> platforms using FIQs will not have FIQs delivered if they're idle
> (because the swapper will have FIQs masked at the CPU.)

You're right.

What about moving local_fiq_enable() to trap_init() then?


Nicolas
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